1. Technical Field
Embodiments described herein generally relate to a composite structures and, more specifically to composite structures having a smooth surface.
2. Background
Composite structures are used in a variety of applications and have become increasingly common in consumer products. For example, composite structures may serve as housings for electronic devices, such as a notebook computing device housing. However, achieving a high quality outer cosmetic layer in a composite structure can be challenging.
Generally, composite structures, such as a notebook computing device housing, may include multiple layers of unidirectional carbon/epoxy pre-impregnated fiber weave (“prepeg”), such as epoxy used to make carbon fiber reinforced plastic (CFRP). The prepeg is placed in a mold and cured under heat and pressure. The heat causes the viscosity of the epoxy to initially drop, often significantly. As the epoxy cures, the viscosity rises and the epoxy solidifies. During the time when the viscosity of the epoxy is low and the structure is under pressure, the epoxy flows to lower pressure areas (e.g., areas where the mold cavity is larger, areas with gaps or voids, out a parting line of the mold, and so on). This resin flow can change the orientation of the reinforcing carbon fibers and “wash” the fibers in the direction of the resin flow, if the flow is sufficiently fast and/or voluminous. In many cases fibers that are carefully oriented during lay-up will be distorted after molding.
Additionally, defects and/or discontinuities on a mold surface may generally show through on the other side of the structure. This is referred to as “print through.” For example, a circular steel plug in a mold surface impacting one side of a structure will generally create a faint circular pattern on the opposite wall of the structure. Similarly, a structure that transitions from four plies to eight plies will typically show four transition lines on the front side of the structure. Ply joints may also print through to a surface layer.
In some cases, it may be desirable to make structures with different materials in specific locations. For example, some computer housings generally made from a carbon and epoxy material use glass/epoxy “windows” in certain parts of the housings to enhance the performance of an antenna behind the window. The transition joint between the glass/epoxy window to the carbon and epoxy housing will generally print through to the housing front side due to thickness differences between the materials, as well as the aforementioned transition issues.
It also may be difficult to achieve a defect-free cosmetic surface when molding composite panels with a foam, balsa, or honeycomb core. This may be due to the core material having inconsistent thickness, absorbing resin at a non-constant rate, uneven pressure applied across the material and so forth.